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RCS

Application-orientated Robot-based Precision Antenna Measurement System
Jae-Yong Kwon, Woohyun Jung, Sangsu Lee, October 2024

KRISS has introduced a 6-axis industrial robot into an antenna measurement system. This allows for various measurement applications and the continuous development of additional ones. Among these measurement applications, representative functions such as antenna gain measurement, material characteristics measurement, 3D field scanning, and RCS measurement are continuously being improved. The validity of the technology is discussed by comparing its results with measurements taken in a fully anechoic chamber.

Monostatic Measurement Setup and Transformation Method to Obtain Bistatic Reflection Patterns of Reconfigurable Intelligent Surfaces
Fabian T. Bette, Thomas M. Gemmer, Severin von Wnuck-Lipinski, Hendrik Bartko, Benoit Derat, Simon Otto, Maren Willemsen, Wilhelm Keusgen, October 2024

To verify the proper working of a Reconfigurable Intelligent Surface (RIS), similar to antenna radiation patterns, the RIS reflection pattern has been established as key performance indicator. To overcome the necessity of a bistatic RIS qualification setup, where two antennas at different positions are used, this paper presents a novel measurement approach to obtain the RIS reflection pattern based on a monostatic indirect Far-Field (FF) Compact Antenna Test Range (CATR) setup. Due to the monostatic principle, only one antenna, which is used for transmission and reception, is required. Subsequently, the mono- static reflection patterns are transformed into bistatic reflection patterns by applying different Monostatic to Bistatic Equivalence Theorems (MBETs) known from radar-cross-section theory. With that, the required setup can be simplified in terms of mechanical complexity, setup footprint and the number of measurement scenarios, since incident and reflection angle correspond in the monostatic case. This paper analyzes three different MBETs, namely Kell, Crispin/Siegel and Falconer, with respect to their suitability for RIS reflection pattern measurements. Moreover, a monostatic CATR test environment is presented and two metal plate based RIS calibration approaches are introduced. This novel monostatic RIS measurement approach is validated with simulation and measurement data of two mmWave fixed beam RISs. Both of them are reflecting an impinging signal from broadside (θ = 0°) direction into 47° at a center frequency of 27GHz. The results prove the suitability of this approach.

Validating the Conductive Resonant Sphere Creeping Wave Phase Dilation
Donald P. Hillard, Michael S. Emire, Michael D. Safty, Richard W. Soard, Gary Salvail, Robert C. Simpson, October 2024

This paper presents research validating the conductive resonant sphere creeping wave phase dilation discovered in high-resolution imaging presented at the 2023 Antenna Measurement and Techniques Association (AMTA), which focused on using a small resonant sphere as a test probe for assessing Radar Cross Section measurement accuracy [1]. The associated analysis uncovered a discrepancy in the creeping wave Standard Model physical pathlength around the sphere having less phase than required for resonance. This paper presents a new creeping wave phase dilation model resolving the phase difference and validating results with computational electromagnetic field predictions.

Estimating the Monostatic RCS of Variable Ratio Pylons Using MoM with Localized Meshing
Mark Ingerson, Vince Rodriguez, October 2024

Larger low-observable targets are being mounted onto RCS pylons. In many cases not only Azimuth rotation of the target, but a degree of movement in elevation is desired. This requires in many cases a large number of positioning cables to run from the base of the pylon to the tip where the rotator is placed. At the same time the low-observable qualities of the target call for pylon ogives with higher ratios to minimize the background RCS of the pylon that supports the target. The higher ratios call for very thin structures that cannot handle the weight of the rotator or have not enough space for the control and power cable to be fed to the rotator. A way of solving this problem is to have a variable ratio pylon, where the ogive at the tip is different from the ogive on the main body of the pylon. To analyze these pylons a higher-order basis-function method of moments (HOBFMoM) approach has been used in the past [1]. To conform the quadrilateral flat patches to the round geometry of the pylon, patches smaller than 0.3λ were used. While this was still an advantage over the typical 0.1to 0.05λ patches it placed limits on the highest frequencies that could be analyzed give the available computational resources. In this paper the authors present an approach to the meshing of the structure that allows for computing the monostatic RCS at frequencies in the x-band for a 2.4 m tall pylon. In addition, the effects of the non- physical absorber terminations are further analyzed.

Design and Demonstration of a Low-Cost Radar Cross-Section Range for Measurements of Wideband Van Atta Arrays
Songyi Yen, Ljubodrag B. Boskovic, Dejan S. Filipovic, October 2024

A low-cost, custom radar cross-section (RCS) range is designed and built to measure the RCS of wideband linear and circular retrodirective arrays over a ground plane and good accuracy over 1-4 GHz is demonstrated. The only test equipment needed is a vector network analyzer (VNA). The 3.66 m × 1.22 m ground plane is constructed from a thin aluminum foil covering a frame constructed out of plywood and foam panels with wideband probe and wideband arrays inset into the ground plane. Excellent agreement with theoretical and simulated results is demonstrated. Additionally, the measurements validate a previously proposed method for the synthesis of the antenna component of the RCS using only the scattering parameters and embedded element patterns. Extension of this synthesis method into the measurements for the case when no beamforming network is available is demonstrated as well. Time-domain RCS measurements also agree well with theoretical and simulated data, which are used to illustrate the physics of linear Van Atta arrays.

Radar Echoes from Dielectric Strings supported Objects
Pax Samuel P. Wei, October 2024

At the Boeing 9-77 Range, we have used various dielectric strings and fishing ropes for target support. An advantage of a string-system is that when not at broadside to the incident wave, the strings would give rise to the least interference to the object being measured. A good example is a 60-ft long rod lifted from ground by the upper turntable (UTT) to the quiet-zone center, rotated horizontally and being measured. [1-2] For the long rod with abruptly terminated ends, there is often a ringing, called the Gibbs phenomenon, which modulates the responses at both ends. Yet, for a 40-ft long vertical metal cylinder supported by a rope through its center, it was curious that the ringing did not show up. By reviewing the metal-dielectric interference, we now realized that the dielectric rope must have contributed an opposite effect such that the ringing ceased. These results are described and discussed. [3-4]

Advanced Signal Processing Technique for Enhancing RCS Measurements in High-Noise Environments
Papa Ousmane Leye, Daria Kulikova, Ming Dong, Chaouki Kasmi, Felix Vega, Islem Yahi, October 2024

Measuring radar cross-section (RCS) in high-noise environments remains a challenge. This paper presents an advanced signal processing framework that uses statistical dimensionality reduction to effectively separate the signal of interest from environmental noise. The proposed technique consists of two main steps. First, background subtraction and a gating technique are used to preprocess the measured data, separating and extracting the target’s reflectivity distribution from unwanted room contributions. Then, principal component analysis (PCA) is employed to analyze the target’s scattering image and localize its main scattering centers. To validate the proposed algorithm, a perfectly electrically conductive (PEC) scaled UAV model is manufactured and tested. The analysis of the experimental results demonstrates that the suggested technique effectively removes background and clutter, providing reliable RCS measurements in noisy environments.

Background and clutter removal algorithm for RCS extraction in semi-anechoic chamber
Papa Ousmane Leye, Adamo Banelli, Shaikha Aldhaheri, Chaouki Kasmi, Felix Vega, Islem Yahi, October 2023

The purpose of radar cross-section (RCS) measurement is to determine the amount of scattering that occurs when the radar signal illuminates the target. It is generally performed to prove a design concept. RCS measurement chamber requires a good signal-to-noise ratio during the measurement. When the measurement is performed in a non-controlled environment, coherent background subtraction associated with time gating is commonly used to improve the quality of the RCS data. Although these techniques are usually effective, residual clutter and background level still need to be removed to accurately characterize the target’s RCS in highly cluttered environments, such as semi-anechoic chambers. In this paper, a four-step post-processing technique is presented. In addition to the vector background subtraction and timegating techniques implemented in our previous work, a statistical algorithm called Principal Component Analysis (PCA) is applied to the ISAR image of the target. It is an extension of the PCA technique to RCS measurement. It is shown that residual background and clutter can be reduced by the statistical filtering method through eigenvalue decomposition of the RCS data. The technique is presented and evaluated through measurement of the RCS of a dihedral corner reflector at the X-band in the semi-anechoic chamber of the Directed Energy Research Center.

Background and clutter removal algorithm for RCS extraction in semi-anechoic chamber
Papa Ousmane Leye, Adamo Banelli, Shaikha Aldhaheri, Chaouki Kasmi, Felix Vega, Islem Yahi, October 2023

The purpose of radar cross-section (RCS) measurement is to determine the amount of scattering that occurs when the radar signal illuminates the target. It is generally performed to prove a design concept. RCS measurement chamber requires a good signal-to-noise ratio during the measurement. When the measurement is performed in a non-controlled environment, coherent background subtraction associated with time gating is commonly used to improve the quality of the RCS data. Although these techniques are usually effective, residual clutter and background level still need to be removed to accurately characterize the target’s RCS in highly cluttered environments, such as semi-anechoic chambers. In this paper, a four-step post-processing technique is presented. In addition to the vector background subtraction and timegating techniques implemented in our previous work, a statistical algorithm called Principal Component Analysis (PCA) is applied to the ISAR image of the target. It is an extension of the PCA technique to RCS measurement. It is shown that residual background and clutter can be reduced by the statistical filtering method through eigenvalue decomposition of the RCS data. The technique is presented and evaluated through measurement of the RCS of a dihedral corner reflector at the X-band in the semi-anechoic chamber of the Directed Energy Research Center.

A Squat Cylinder-Dihedral Dual Calibration Device for Compact Ranges at UHF
Hirsch Chizever, Laura Suzuki, October 2023

The use of squat cylinders as both primary and secondary calibration targets is commonplace within the radar cross section (RCS) measurement community. Secondary calibrations have become a best practice activity for ranges seeking or maintaining certification. The calibration process, often referred to by the measurement community as a “Dual-Cal,” uses two squat cylinders of similar but unequal dimensions that provide range operators with a broadband calibration vector and a measurement uncertainty metric important to range certification. Despite their popularity, the need to ensure resonance scattering occurs below the desired measurement band results in physically large cylinders at UHF. In addition, the need to access the test zone for separate cylinder measurements may add substantial time to the calibration process and require specialized equipment, especially for large ranges. In response to these issues, a 22.5-degree right dihedral has been inserted into a squat cylinder form factor, creating a primary and secondary calibration target within one body, each separated in azimuth by 180 degrees. This two-target calibration device removes the need to access the target zone twice and mitigates errors associated with separate mounting schemes. The cylinder aspect, now truncated by the imposition of a dihedral, has 50% extended lower frequency coverage at UHF due to oblique edge scattering at vertical polarization. At horizontal polarization, the dihedral interruption of the cylinder creeping wave reduces its contribution for ka<4. The dihedral aspect provides a full polarimetric calibration, resulting in co-equal frequency responses for each polarization in the high frequency limit. The design parameters of the squat cylinder-dihedral device, its computed full-wave frequency response, and relevant scattering features are discussed.

Measurements on extended long objects for radar field probes
Pax Wei, October 2023

In a compact range when the antenna is used for both transmitting and receiving in a monostatic fashion, the wave packet senses everything within its view. An extended long object usually gives rise to a bright reflection (glint) when viewed near its surface normal. To take advantage of this phenomenon, a discrete Fourier transform (DFT) on RCS measurements would yield a spectrum of incident wave distribution along that object, provided the scattering property is uniform along its length. Compared with traditional field-probes which translate a sphere across the test zone in horizontal and vertical directions, this new method extends out from the usual quiet zone, and is faster and less interfering to the field being probed. Inspired by this idea, the progression to practical innovation is discussed.

Compact RCS Test Range Feed Carousel and Baffle House Design
Gil Yemini, Stefano Sensani, Andrea Giacomini, Lars Foged, Marcel Boumans, Matan Kahanov, Maria Baskin, Ilan Kaplon, October 2023

A new compact range for RCS measurements has been installed and qualified by Orbit/FR Engineering Ltd. MVG. It has a Quiet Zone of 3m diameter, 3m length and operates from 0.7 to 50 GHz, with a feed carousel that allows for fully automated feed change. The RF design is not intended for antenna measurements in its current configuration, but mainly dedicated to RCS. The operational frequency band is split into three sub-bands: each of the lower two bands have a monostatic operated dual polarized feed, while the higher band has a quasimonostatic operated feed configuration with two dual polarized feeds. Pulsed Tx/Rx modules are directly integrated into the feed assembly. Also, the RF band switching equipment, as well as the network analyzer, are integrated in the feed carousel, so that there are no flexing cables or any other relative movement of RF components when the relevant feed is moved into the focus. Together with tight temperature control, this leads to the best possible RF stability. Since all measurements are time gated, there is no need for an absorber baffle wall to prevent feed direct leakage into the quiet zone. Thus, all feeds are mounted on a clean absorber disk without any absorber blockage and unwanted primary pattern distortion down to a conical angle of 90deg. This allows to obtain an exceptionally good QZ performance even at the lowest frequencies, with an outstanding comparison with the predictions based on Physical Optics. The paper will describe the range design fundamentals, the feed carousel concept and the relevant RF instrumentation. The Quiet Zone performance evaluated by field probing with a Shorted Antenna located in the Quiet Zone will be extensively presented, demonstrating full compliance with the specifications.

Compact RCS Test Range Field Probing using a Shorted Antenna as Target
Gil Yemini, Stefano Sensani, Andrea Giacomini, Lars Foged, Marcel Boumans, Matan Kahanov, Maria Baskin, Ilan Kaplon, October 2023

A new compact range for RCS measurements has been qualified. It has a quiet zone of 3m diameter, 3m length and operates from 0.7 to 50 GHz. The range is oriented for RCS measurements, whereas antenna measurements are not foreseen. All RF equipment is integrated close to the feeds with highly integrated pulsed Tx/Rx-modules. Therefore, classical field probing by moving a probe antenna along a linear slide would require significant modification of the RF system. If one measures the RCS of a target on the linear slide, it is difficult to distinguish the target down range reflection from the reflection of the linear slide structure. A long stand-off between target and slide is not practical for mechanical reasons in regard to accuracy requirements at 50 GHz. More important, simply measuring a reflective plate will not give any cross-polarization information. A more advanced target is created by using an antenna with a short circuit after an RF cable to locate the reflection of the short well behind the scanner in down range. In addition, the antenna receives only nominal quiet zone co-polarization, consequently, only reflects co-polarization from the short, and the feed receives the compact range induced cross-polarization at the feed (oneway). The method has shown to be extremely effective. More important, it uses the RF instrumentation and RCS measurement methods as designed for regular operation without any modification, thus is the most realistic system level quality representation of the quiet zone, can be repeated at any time without elaborate range reconfiguration requirements and can serve as part of the commissioned RF system performance qualification. The paper will present the quiet zone field probe test setup, a calculation of antenna and RF cable requirements, an analysis of the down range profile of scanner and reflective antenna and field probing results.

Reproducible Measurements of “Fan Blades in a Pipe” CEM Benchmark
Jon Kelley, Kurt Norris, Brian Mackie-Mason, Brody Barton, David Chamulak, Scott Schaeffer, Mark Martin, Kendall Crouch, Clifton Courtney, Ali Yilmaz, October 2023

—Cylindrical hubs with fan blades are inserted into a pipe inside a modified camera box—a recently introduced structure intended to host differently-shaped ducts behind an aperture. The resulting structures increase the reproducibility of commonly used simplified jet-engine inlet models and are designed to serve as precisely-defined radar cross section (RCS) benchmarks with reliable reference results. The design, manufacturing, and assembly of the measured structures are detailed; the RCS measurement setup, data collection, and post processing are documented; and the uncertainty in measured RCS data is quantified with the help of simulations. Results show that the fields scattered by the structures, while highly sensitive to geometric and material perturbations, can be both measured and simulated accurately even at frequencies with many propagating modes inside the pipe.

Compact Bistatic Radar Cross Section Measurement System Using a New Plane-Wave Synthesis
Masanobu Hirose, Satoru Kurokawa, October 2023

We propose a compact bistatic radar cross section (RCS) measurement system using a new 2D plane-wave synthesis (PWS) employing 2D propagating plane-wave expansion and a single-cut near-field far-field transformation (SCNFFFT). Our system has been successfully applied to the bistatic RCS measurements of a metasurface (100 mm width, 50 mm height, and 0.127 mm thickness) at 60 GHz where two horn antennas are used for the PWS (Tx) and the SCNFFFT (Rx) and placed at the circular distances of 1.735 m and 0.35 m respectively. The peak and pattern errors of the RCS are 0.4 dB and below -25 dB respectively. Using the proposed 2D PWS and SCNFFFT, the compact 2D bistatic RCS measurement system is realized without large equipment such as CATR.

The Small Resonant Sphere for Validating Radar Cross Section Measurement Accuracy
Donald Hilliard, Michael Emire, Long To, October 2023

This paper presents research results conducted at the Naval Air Warfare Center Weapons Division (NAWCWD) Radar Reflectivity Laboratory (RRL) to characterize RCS measurement quality of a compact range anechoic chamber using a small resonant sphere as a test probe measured over a 3.17-octave bandwidth, which covers the first half of the resonance region. Specifically, tests were performed on 1-inch and 12-inch diameter spheres over 2-18 GHz, which is a very prevalent test spectrum for RRL customers. The spheres were tested at the quiet zone center and the 1-inch was rotationally scanned over a 1- meter radial arc within the test zone. Spectral and spatial analysis was performed using techniques developed by Dr. Dean L. Mensa [1].

ISAR Image Gating Using Backprojection and Smoothed Reweighted L1-optimization
Christer Larsson, Andreas Gällström, October 2022

Inverse Synthetic Aperture Radar (ISAR) image gating for RCS extraction using backprojection is compared with image gating using smoothed reweighted L1-optimization in this study. The RCS of an object is measured by placing the object placed on a turntable which is rotated in an angular range while sweeping the frequency in the desired frequency range. A common model with isotropic point scatterers fixed in the object coordinate system is used in the ISAR imaging process. This model is used to define a forward operator. The ISAR image can be formed by operating with the backpropagation operator (i.e. backprojection), the adjoint of the forward operator, on the measured RCS. This robust method to solve the inverse problem gives an image with a resolution limited by the frequency bandwidth and the angular range. The RCS for a scattering feature is commonly determined by using the forward operator on the point scatterers in the image that are determined to belong to the scattering feature in ISAR image gating. L1-optimization is a method that can be used to get images with higher resolution and hence better separation of the different scattering features than backprojection. L1-optimization is well suited for naturally sparse ISAR images. One method to mitigate that the scatterers are restricted to a fixed grid is to use smoothed reweighting [1]. L1-optimizations are performed consecutively in a few steps where a smoothed version of the previous solution is used to determine a weighting matrix for the next step. Smoothed reweighted L1-optimization gives images with better separation of the scattering features in the ISAR image. Simulated and measured RCS data are used to compare image gating using backprojection with gating using smoothed reweighted L1-optimization. The main conclusion of this study is that the RCS can be extracted for scattering features, not resolved in backprojection images, using the smoothed reweighted L1-optimization. [1] D. Pinchera and M. D. Migliore, “Accurate reconstruction of the radiation of sparse sources from a small set of near-field measurements by means of a smooth-weighted norm for cluster-sparsity problems,” Electronics, vol. 10, no. 22, p. 2854, 2021.

Stepped-Frequency CW RCS measurement in Semi-Anechoic Chamber
Papa Ousmane Leye, David Martinez, Shaikha Aldhaheri, Chaouki Kasmi, Nicolas Mora, October 2022

The RCS of a target can be estimated using electromagnetic modeling if accurate geometries and material descriptions are available. An exact numerical calculation often requires prohibitive processing times. Moreover, numerical predictions with approximate techniques are difficult as it is challenging to consider all the physical phenomena. Therefore, a suitable RCS measurement facility adapted to the target size and specifications is required to estimate the RCS of a given target and to validate the numerical predictions. In general, the measurement of RCS takes place in anechoic chambers that simulate free-space and far-field conditions and where the unwanted reflections (walls, target mount, objects in the range, and the target interactions) are reduced. This paper presents a broadband measurement and validation of the RCS of a metallic trihedral corner reflector of 30 cm sides when fully anechoic conditions are not available, and consequently, some undesirable echoes are present in the measurements. Firstly, the measurement facility calibration and the target calibration are outlined. A single target reference approach is performed using a sphere as a reference, and its scattering response is shortly described. Then, the measurement of the target is performed. After these steps, a processing procedure is applied to isolate the target response from the background and the close responses due to unwanted reflections. The post-processing technique and the acquisition system are presented and discussed. The measurements are performed at X band as a function of the viewing angle for vertical transmit and receive polarization. To validate the technique, the RCS of the trihedral corner reflector is numerically simulated using the Integral Solver (I-Solver) of CST, with a Gaussian excitation, for vertical transmit and receive polarization. Measurements are compared with results obtained from CST software and show a good agreement with the numerical simulations. This setup will be used for RCS measurement of different complex targets and compared with measurements from other facilities to analyze and evaluate the RCS measurement uncertainty.

Experimental validation of Plane Wave Generator for 5G New Radio FR2 applications
Shoaib Anwar, Evgueni Kaverine, Fabien Henry, Nicolas Gross, Francesco Scattone, Darko Sekuljica, Andrea Giacomini, Francesco Saccardi, Alessandro Scannavini, Per Iversen, Lars Foged, October 2022

Plane wave generator (PWG) for Over The Air (OTA) characterization of beamforming millimeter wave devices, provides an attractive solution comparing to conventional measurement techniques (Compact Antenna test Ranges (CATR) and Far-field chambers). MVG’s Plane wave generator for 5G NR FR2 applications ([1]-[4]) is an innovative tool which permits the user to measure the radiating elements with low to medium directivity radiation characteristics with excellent precision. Conventional CATR systems are not suited for stationary DUT (with / without person) measurement scenario. In this paper, experimental results are presented for a dual-polarized PWG system, covering the 3GPP bands n257, n258 and n261 (24.25-29.5 GHz). System measurement results show good comparison with simulations and measurements of the PWG alone. Another advantage of PWG presented here, is that we can modify the size of the QZ. Results from a pre-production unit for a 15cm QZ shows amplitude variation of less than ±1 dB and achieve more precision for smaller DUT. Measurement results from the pre-production unit with a quiet zone of up to 38cm sphere diameter, show amplitude variations of less than ±2dB. This variation is compatible with the DUT + phantom or human measurement application. Pattern results for Antenna Under Test (AUT) with low to medium directivity (6dBi up to 17dBi) compare well with simulations and measurements from other systems. For a given AUT, the impact of different positioning mast is also evaluated. Excellent stability of patterns, when the AUT is placed at different positions inside the QZ, is observed. These results confirm that the dual-polarized PWG system presents an attractive solution for FR2 characterization of low to medium directivity radiating elements.

Ground Penetrating Radar Antenna Evaluation
Joseph Friedel, David Oyediran, David Rohde, October 2022

The mission of the Naval Surface Warfare Center, Indian Head, Maryland, EOD Department, is to utilize the latest available technology in the advancement of Explosive Ordnance Disposal (EOD) equipment and techniques. This mission includes the test and evaluation of current and developmental systems, which will be discussed in this paper. EOD exploits multiple physical phenomena in its task of ordnance detection, including chemical and electromagnetic. Electromagnetics include RF fields, light (including laser, infrared and ultraviolet), and nuclear radiation. For each phenomena, there may be several different technologies used to provide multi-mode detection capability. This study focuses on the electromagnetic subset of detection RADAR, and specifically Ground Penetrating Radar (GPR), which is distinguished by its earth surface domain and generally downward field of view. The paper will give a very brief overview of GPR theory and equipment, its use in EOD, and then will focus on the RF test and measurement of electromagnetic fields generated by GPR systems and antennas. An RF antenna/system test plan will be detailed, along with the design and development of antenna gain and radiation pattern measurement techniques. The measured data from GPR technology will be graphically displayed, analyzed and compared in terms of the potential for GPR effectiveness.







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